Example:

•http://www.agleader.com/products/directcommand/optrx-crop-sensors/

•http://www.agleader.com/products/ 

 Use the GreenSeeker handheld to instantly take a reading of your crop's health. Readings can be used to make non-subjective decisions regarding the amount of fertilizer to be applied to your crop, resulting in more efficient use of fertilizer—a benefit to your bottom line and the environment.

How it works
*The sensor emits brief bursts of red and infrared light, and then measures the amount of each type of light that is reflected back from the plant

*The sensor continues to sample the scanned area as long as the trigger remains engage.

*The sensor displays the measured value in terms of an NDVI reading (ranging from 0.00 to 0.99) on its LCD display screen

*The strength of the detected light is a direct indicator of the health of the crop; the higher the reading, the healthier the plant

Use the Connected Farm™ scout app on a smartphone or tablet to calculate fertilizer application rates from crop readings taken with the GreenSeeker handheld.

Reference
Govaerts, B., & Verhulst, N. (2010). The normalized difference vegetation index (NDVI) Greenseeker (TM) handheld sensor: toward the integrated evaluation of crop management part A: concepts and case studies.
https://www.trimble.com/Agriculture/gs-handheld.aspx?tab=Product_Overview

https://agriculture.trimble.com/precision-ag/products/greenseeker/

 The SPAD-502 meter is a hand-held device that is widely used for the rapid, accurate and non-destructive measurement of leaf chlorophyll concentrations. It has been employed extensively in both research and agricultural applications, with a range of different plant species. However, its utility has not been fully exploited in relation to the most intensively studied model organism for plant science research, Arabidopsis thaliana. Measurements with the SPAD-502 meter produce relative SPAD meter values that are proportional to the amount of chlorophyll present in the leaf.
https://www.researchgate.net/publication/49711984_Use_of_a_SPAD-502_meter_to_measure_leaf_chlorophyll_concentration_in_Arabidopsis_thaliana

Discover an optimum, balanced crop management system and learn how your business and the environment can benefit. The ISARIA system will add precision to your crop farming management, providing for optimum use of the yield potential of your fields, and increasing the quality of your products and economic efficiency at the same time.


Calculating

Application

Benefits

http://fritzmeier-umwelttechnik.com/isaria/?lang=en
Retrieved at 21 January 2019

The measurement of the chlorophyll content is made with the living plant without damaging it and quickly. clorofiLOG allows monitoring the nutritional status of the plant during its development, not only between one crop and another.

The chlorophyll content in the leaves of the plant is proportional to the amount of nitrogen it has absorbed. Thus, with clorofiLOG it is possible to know areas of cultivation that are deficient in nitrogen and to correct fertilization. Or, you can know where the nitrogen levels are already satisfactory and avoid unnecessary waste of fertilizer. 

With unique technology, clorofiLOG uses 3 waveleghts of light, allowing detailed analysis and accurate and fast measurements. These measurements can be instantly visualized or stored for computer analysis. The optical measurement analyzes the absorption of light by the leaf, indicating the presence of chlorophyll.

Much of the pollution generated by agriculture is felt by the high levels of nitrates in rivers. So it is important to make rational use of fertilizers, avoiding waste that is not used by plants and end up polluting the soil and rivers. With clorofiLOG the producer can dispense the correct amount of fertilizers in agriculture and help reduce this type of contamination in the water, preserving the environment.


-https://www.google.com/search?q=ClorofiLOg+Brazil&oq=ClorofiLOg+Brazil&aqs=chrome..69i57.9663j0j1&sourceid=chrome&ie=UTF-8
-http://www.falker.com.br/en/product-clorofilog-chlorophyll-meter.php

>Developed at Yara’s R & D centre in Germany, The N-Sensor is mounted on the front of the tractor cab and measures the nitrogen demand using crop specific light reflectance. The N-Sensor bases its measurements on specific wavelengths of light to establish exact levels of biomass and colour within the leaf. The data is transferred to the spreader which adjusts the application rate accordingly, thus reducing the environmental effects of run-off and maximising yield. It has been in use on arable farms since 1996. The Grass Sense project has taken the N-Sensor and conducted trials on UK grassland, and has shown that uniform application of nitrogen fertiliser results in overand under-fertilisation with optimum N rates for 1st cut silage varying substantially from 60 to 165 kg N/ha (UK trial). The trials have shown N uptake in grassland can be predicted very accurately which has resulted in the release of the N-Sensor grass calibration.